As described in Japanese Patent Application Publication No. 2002-198322, a flash lamp is used so as to heat, at high speed, a substrate for ion implantation in a surface of a semiconductor wafer or impurity activation therein. Moreover, as described in Japanese Patent Application Publication No. 2005-527972, in another conventionally known technology, preheating is performed in addition to such rapid heating by using a halogen lamp. An advantage of using a flash lamp for heating a substrate is that it is possible to suppress excessive diffusion of the impurities to be activated, in the depth direction of a semiconductor, since heating is performed quickly so that the process can be completed for a short time.
In a general flash lamp, a pair of main electrodes (E1, E2) for flash discharge light emission i.e., main discharge, is arranged to face each other in an electrical discharge space (Ds), and a starting electrode (Et) is provided outside the lamp bulb so as not to be in contact with the electrical discharge space (Ds). A lighting circuit therefor has a main capacitor (Cz) for accumulating electric charges for main electric discharge, a charge circuit (Ux) for charging the main capacitor (Cy), and a starting circuit (Ut), which impresses high voltage to the starting electrode (Et). The main electrodes (E1, E2) of the lamp are connected to the respective (both) terminals of the main capacitor (Cz), wherein when the starting circuit (Ut) is activated in a state where charging of the main capacitor (Cz) is completed, the high voltage is impressed to the starting electrode (Et), so that dielectric barrier discharge occurs in the electrical discharge space (Ds) of the lamp, whereby main discharge is induced and generated by plasma produced due to this electric discharge between the main electrodes (E1, E2), so that the lamp can be turned on.
Although voltage of the main capacitor starts decreasing since the energy accumulated in the main capacitor is released from the lamp when the main discharge starts, lamp current gradually increases, reaches a peak, and then starts gradually decreasing since an increase rate of the lamp current after the start of the main discharge is controlled to fall within a predetermined range by usually inserting an inductor in a path connecting the main capacitor and the lamp. If the voltage impressed to the lamp from the main capacitor becomes less than the minimum voltage value, at which electric discharge can be maintained, or if the current, which flows into the lamp, becomes less than the minimum current value, at which electric discharge can be maintained, electric discharge stops so that the lamp goes out.
Various proposals for measures and improvements of such a heating apparatus and annealing apparatus are made to practically use substrate heating, in which a flash lamp is used. For example, Japanese Patent Application Publication No. 2009-164080 discloses proposed technology, in which when simmer discharge is generated on a lower side in gravity just before main discharge to extend the life-span of a lamp, and electric discharge floats up even near the arc tube axis due to heat convection in an arc tube, a simmer discharge resistor is short-circuited by a switching unit to generate main discharge, whereby the main discharge current is suppressed from flowing into the inner wall of the arc tube, or a portion near there, thereby preventing turbidity and breakage of the arc tube.
Moreover, to shorten heating time, when a rise of light emission of a flash lamp is made steep so that the temperature of a substrate rises steeply, there is a problem on which deformation or cracks of the substrate occurs, due to heat strain generated by the difference in temperature of a bottom face and a surface of the substrate. In Japanese Patent Application Publication No. 2009-164201, the technology for avoiding such a problem is proposed. In the technology, it has been found that it is desirable to perform a processing in which the surface temperature of the substrate is not raised at a stretch to target temperature, but it is temporarily raised to second temperature lower than the target temperature, thereby holding that temperature for a short time, or the temperature thereof is raised while the temperature rise rate is controlled, and then it is raised to the target temperature. To realize the technology, a semiconductor switch 25 is connected in series to a flash lamp, and after the flash lamp is triggered, the semiconductor switch 25 is turned ON/OFF at least once. Then the semiconductor switch 25 is turned ON only once, so that the waveform of current, which flows into the flash lamp, is controlled.
However, in the technology described in Japanese Patent Application Publication No. 2009-164201, there is a problem that a desired lamp current waveform cannot be realized. In case where a period of low lamp current is set to a low level, the problem is attributed to a possibility that electric discharge stops and the light goes out when it becomes, within the period, less than the minimum current value, at which electric discharge can be maintained in the lamp, due to manufacturing tolerance and temperature conditions of a lamp, and/or fluctuation of timing of ON/OFF of the semiconductor switch 25.
To re-light the lamp when the light goes out, a considerable time delay for re-lighting the lamp is needed, since it is necessary to wait until whether the lamp goes out is detected so that a signal for activating the starting circuit (Ut) is outputted, whereby the starting circuit (Ut), which receives the signal, starts an operation to generate high voltage, and dielectric barrier discharge is produced in the electrical discharge space (Ds) of the lamp by the impressed high voltage, thereby causing the main discharge between the main electrodes (E1, E2). Moreover, there are variations in timing from when the high voltage is generated by the starting circuit (Ut) until electric discharge actually starts. Thus, they adversely affect a heating program in substrate processing.
Therefore, lamp current can be only reduced to a current value, in which a sufficient amplitude is available with respect to the minimum current value, so that design and setting of lamp current waveform are restricted.